1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device, and specifically to ESD protection devices, either having an ESD protection function only or having an ESD protection function and other functions.
2. Description of the Related Art
ESD is a phenomenon in which strong discharge is generated when a charged conductive body (e.g., human body) comes into contact with or comes sufficiently close to another conductive body (e.g., electronic device). ESD causes damage or malfunctioning of electronic devices. To prevent this, an excessively high voltage generated during discharge must be prevented from being applied to circuits of the electronic devices. ESD protection devices used for such an application are also called surge absorbers.
An ESD protection device is disposed, for example, between a signal line and a ground of a circuit. The ESD protection device includes a pair of discharge electrodes that face each other with a space provided therebetween. Therefore, the ESD protection device has high resistance under normal operation and a signal is not sent to the ground. When an excessively high voltage, for example, generated by static electricity through an antenna of a cellular phone is applied, discharge is generated between the discharge electrodes of the ESD protection device, which leads the static electricity to the ground. Thus, a voltage generated by static electricity is not applied to the circuits disposed downstream from the ESD protection device, which protects the circuits.
For example, an ESD protection device shown in an exploded perspective view of FIG. 9 and a sectional view of FIG. 10 includes a cavity 5 provided in a ceramic multilayer substrate made by laminating insulating ceramic sheets 2. Discharge electrodes 6 that face each other and that are electrically connected to external electrodes 1 are disposed in the cavity 5 that includes discharge gas. When a breakdown voltage is applied between the discharge electrodes 6, discharge is generated between the discharge electrodes 6 in the cavity 5, which leads an excessive voltage to the ground. Consequently, the circuits disposed downstream from the ESD protection device are protected (see, for example, Japanese Unexamined Patent Application Publication No. 2001-43954).
In this ESD protection device, the discharge electrodes disposed on both sides of the discharge space are connected to the external electrodes. Since a certain amount of space or greater is required with such a structure, it is difficult to reduce the size of the ESD protection device. Furthermore, when high voltage static electricity is continuously applied in a repetitive manner, the discharge electrodes are melted and a short circuit occurs between the discharge electrodes. Alternatively, the distance between the discharge electrodes is increased and, thus, a discharge starting voltage is increased.